Ocean Heat Transport and Water Vapor Greenhouse in a Warm Equable Climate: A New Look at the Low Gradient Paradox

Abstract

The authors study the role of ocean heat transport (OHT) in the maintenance of a warm, equable, ice-free climate. An ensemble of idealized aquaplanet GCM calculations is used to assess the equilibrium sensitivity of global mean surface temperature (T) and its equator-to-pole gradient (DT) to variations in OHT, prescribed through a simple analytical formula representing export out of the tropics and poleward convergence. Lowlatitude OHT warms the mid-to high latitudes without cooling the tropics; T increases by 18C and DT decreases by 2.68C for every 0.5-PW increase in OHT across 308 latitude. This warming is relatively insensitive to the detailed meridional structure of OHT. It occurs in spite of near-perfect atmospheric compensation of large imposed variations in OHT: the total poleward heat transport is nearly fixed.The warming results from a convective adjustment of the extratropical troposphere. Increased OHT drives a shift from large-scale to convective precipitation in the midlatitude storm tracks. Warming arises primarily from enhanced greenhouse trapping associated with convective moistening of the upper troposphere. Warming extends to the poles by atmospheric processes even in the absence of high-latitude OHT.A new conceptual model for equable climates is proposed, in which OHT plays a key role by driving enhanced deep convection in the midlatitude storm tracks. In this view, the climatic impact of OHT depends on its effects on the greenhouse properties of the atmosphere, rather than its ability to increase the total poleward energy transport.

Changes in the seasonal cycle of the Atlantic meridional heat transport in a RCP 8.5 climate projection in MPI-ESM

“…In MPI-ESM, the atmosphere compensates for the decrease in the meridional ocean heat transport, implying an increased atmospheric heat transport (not shown), as also suggested by Rose and Ferreira (2012). A deeper analysis of the atmo- spheric compensation and changes in the atmospheric heat transport is needed but is beyond the scope of our study.…”

The Role of Oceans and Sea Ice in Abrupt Transitions between Multiple Climate States

“…FMR11 showed that enhanced OHT into midlatitudes stabilizes the large ice cap at equilibrium. The role of OHT in the equilibrated Warm state is more subtle (Rose and Ferreira 2013). Direct OHT convergence at the poles is small, except during transient cooling.…”

“…OHT at mid-to-high latitudes also scales with c res , collapsing as ice advances and resuming as ice retreats. There is little evidence that OHT scales with w Ek in the subpolar gyre, as assumed by Rose and Marshall (2009).…”

“…FMR11 conclude that the meridional structure of OHT (in particular the tendency of the oceans to warm the midlatitudes) is a key ingredient in the maintenance of multiple states. This result was anticipated by Rose and Marshall (2009) Our paper is laid out as follows: The model and imposed forcing are described in section 2. In section 3 we compare the shapes of the hysteresis loops generated in three different model configurations (Aqua, Ridge, and a simple slab ocean).…”

“…Pressure coordinates are used in the vertical with one level in the boundary layer, three in the free troposphere, and one in the stratosphere. Details about these parameterizations [substantially cruder than those used in Intergovernmental Panel on Climate Change (IPCC)-class models] are given in Rose and Ferreira (2013). Present-day atmospheric CO 2 is prescribed.…”